1. Field of the Invention
This invention relates to a hydrostatic pump with a variable delivery volume which can be operated in a closed circuit. An electrically actuatable control device is provided for the control of the delivery volume of the pump.
2. Description of Related Art
Pumps of the above general type are used as hydrostatic drive systems in a closed circuit to supply hydrostatic traction drives or slewing gear drive systems in mobile work machines. When the pump has an electrically actuatable control device, the pump can be easily incorporated into a vehicle management system.
On known pumps for use in a closed circuit, the electrically actuatable control device has electrically actuatable pilot valves in the form of solenoid valves, by means of which a control pressure can be generated. The control pressure is exerted on a servo device that consists of a slave piston. The slave piston is in communication by means of a linkage with a position-controlled control valve in the form of a pilot valve which controls the application of a positioning pressure to a positioning piston device which is operatively connected to a delivery volume control device such as a pivoting cradle in the form of a swashplate. For this purpose, the pilot valve is in communication on the input side with a reservoir and a control pressure source. The feedback of the position of the delivery volume control device takes place on a movable sleeve of the pilot valve formed by the control valve. With an electrically actuatable control device of this type, the delivery volume of the pump and the delivery direction of the pump can be controlled by an electrical actuation signal to the solenoid valve.
As a result of the presence of the position-controlled pilot valve, on which a mechanical displacement equalization is performed to control the position of the delivery volume control device, and as a result of the presence of the slave piston, a pump of this type with an electrical actuation increases the construction cost of the closed circuit. A pump of this type for a closed circuit with an electrically actuated control device and a mechanical displacement equalization of the position control is described in DE 39 03 189 A1.
On pumps of this type which are used for hydrostatic traction drives in the closed circuit of mobile work machines, for certain applications with a high safety standard, such as the requirement for certification of the mobile equipment under federal motor vehicle safety standards, to ensure that in the event of a defect or malfunction in the electrical control system the pump is pivoted actively and in a controlled manner back into the neutral position, which is also designated as the zero position, in which the pump does not deliver any discharge flow, to achieve a controlled deceleration of the mobile work machine. With a position-controlled pilot valve on known pumps, the pump can be pivoted back into the neutral position that represents the zero position. In addition, it is already known that for such a safety function a restoring spring device can be provided which pivots the delivery volume control device of the pump back into the neutral position. However, a restoring spring device of this type results in high construction costs and takes up a great deal of room on the pump.
Therefore, it is an object of this invention to provide a pump of the general type described above but which is economical to manufacture and is provided in a simple manner with a safety function to ensure that the pump pivots back into the neutral position in a controlled and active manner.